1. Field of the Invention
The present invention relates to an electrostatic chuck that is useful in the production of semiconductor integrated circuits during steps that require a substrate to be held in position, and particularly during ion injection steps, sputtering steps, and plasma etching steps, and to a process for holding a substrate on the electrostatic chuck.
2. Description of the Prior Art
Conventionally, the holding of a substrate during the production steps for a semiconductor integrated circuit has typically employed a wafer chuck that uses either an electrostatic attachment system or a Johnsen-Rahbek system, that is, a so-called electrostatic chuck. Examples of materials that have been proposed for use as the insulating layer for the chuck include plastics such as polyimides, ceramics such as alumina and aluminum nitride, and rubber elastomers such as silicone rubbers.
Furthermore, in a plasma etching step, a cooling mechanism such as a cooling chiller or the like is usually provided on the underside of the electrostatic chuck in order to enable any heating of the wafer caused by the plasma to be suppressed, and enable the temperature distribution of the wafer to be maintained at a uniform level. By maintaining the temperature distribution at a fixed and uniform level, the selectivity of the chuck for the substrates of the masking material and the target material being etched can be improved, and an anisotropic shape can be more readily obtained, meaning more precise etching can be achieved.
Electrostatic chucks with a ceramic insulating layer exhibit excellent durability to plasma gases and have a high level of thermal conductivity. However, ceramic insulating layers are very hard, meaning they develop only poor contact with the irregular wafer surface, and as a result, the contact heat resistance increases, and satisfactory heat dissipation characteristics cannot be obtained. In order to facilitate heat transfer, the usual practice is to pass an inert gas such as helium between the wafer and the insulating layer. However, this method requires fine processing to form grooves within the insulating layer surface to allow the passage of the inert gas, and also requires a device for supplying the inert gas, meaning the structure of the electrostatic chuck becomes significantly more complex, which leads to increases in the production costs for the electrostatic chuck.
Electrostatic chucks that use a polyimide insulating layer are inexpensive and easy to produce, but provide inadequate durability relative to plasma gases. Furthermore, the thermal conductivity is low and the insulating layer is hard, meaning the contact heat resistance is large and the heat dissipation characteristics are poor.
The patent reference 1 proposes an electrostatic chuck comprising a metallic plate, on which is provided a first insulating film comprising a heat-dissipating silicone prepreg obtained by impregnating a glass cloth with a silicone rubber, a copper pattern formed on top of the first insulating film as electrodes, and a second insulating film comprising a silicone rubber formed on top of the copper pattern. Because this electrostatic chuck uses silicone rubber elastomers for the insulating layers, the contact heat resistance is comparatively small, and the heat dissipation characteristics are favorable. Accordingly, using this electrostatic chuck enables the temperature of the wafer to be efficiently maintained at a uniform level. However, in recent years the demands have grown for electrostatic chucks capable of cooling even higher wafer temperatures, and even an electrostatic chuck of the structure disclosed in the patent reference 1 is unable to provide adequate cooling performance.
In addition, the patent reference 2 proposes an electrostatic chuck comprising a first insulating layer, which comprises a heat conductive silicone rubber with a thermal conductivity of at least 0.2 W/mK, and is formed on top of a metal substrate, a conductive pattern which is formed on top of the first insulating layer as electrodes, and a second insulating layer comprising a heat conductive silicone rubber with a thermal conductivity of at least 0.2 W/mK, a hardness measured by a JIS-A hardness meter of no more than 85, and a surface roughness of no more than 5 μm, and this electrostatic chuck exhibits excellent heat dissipation characteristics, and enables the wafer temperature to be precisely maintained at a uniform level. However, with this electrostatic chuck, there is some concern about insulation abnormalities within the second insulating layer caused by factors such as the incorporation of sputtered metals.
[Patent Reference 1] JP59-64245A
[Patent Reference 2] U.S. Pat. No. 6,071,630